Coated paper is used for a large range of products including packaging, art paper, brochures, magazines, catalogues and leaflets. Such coated paper is required to give a range of properties, including brightness, opacity and sheet gloss, as well as printing performance.
Paper coating compositions are generally prepared by forming a fluid aqueous suspension of particulate pigment material together with a binder and other optional ingredients. Lightweight coated, or LWC, paper is generally coated to a coating weight of from about 5 g·m−2 to about 13 g·m−2 on each side, and the total grammage, or weight per unit area of the coated paper is generally in the range of from about 49 g·m−2 to about 65 g·m−2. The coating may conveniently be applied by means of a coating machine including a short dwell time coating head, which is a device in which a captive pond of coating composition under a slightly elevated pressure is held in contact with a moving paper web for a time in the range of from 0.0004 second to 0.01 second, before excess coating composition is removed by means of a trailing blade. However, other types of coating apparatus may also be used for preparing lightweight coated paper. LWC paper is generally used for printing magazines, catalogues and advertising or promotional material. The coated paper is required to meet certain standards of surface gloss and smoothness. For example, the paper is generally required to have a gloss value of at least about 32, and up to about 70, TAPPI units, and a Parker Print Surf value in the range of from about 0.5 μm to about 1.6 μm.
Ultra lightweight coated, or ULWC, paper (otherwise known as light lightweight coated, or LLWC, paper) is used for catalogues and for advertising and promotional material sent through the mail to reduce mailing costs. The coating weight is generally in the range of from 5 g·m−2 to 7 g·m−2 per side. The grammage is generally in the range of from about 35 g·m−2 to about 48 g·m−2.
A very important white inorganic pigment for use in preparing coating compositions for the manufacture of LWC and ULWC papers is processed particulate kaolin clay. Large deposits of kaolin clay exist in Devon and Cornwall, England and in the States of Georgia and South Carolina, United States of America. Important deposits also occur in Brazil, Australia, and in several other countries. Kaolin clay consists predominantly of the mineral kaolinite, together with small proportions of various impurities. Kaolinite exists in the form of hydrous aluminosilicate crystals in the shape of thin hexagonal plates, but these plates tend to adhere together face-to-face to form stacks. The individual plates may have mean diameters of 1 μm or less, but kaolinite particles in the form of stacks of plates may have an equivalent spherical diameter (esd) of up to 10 μm or more. Generally speaking, kaolin clay particles which have an esd of 2 μm or more are in the form of stacks of kaolinite plates, rather than individual plates.
WO-A-99/51815, the disclosure of which is incorporated herein by reference, describes a paper coating pigment comprising a processed particulate kaolin clay the particles of which (i) have a particle size distribution such that at least 80% by weight of the particles have an esd less than 2 μm and not less than 8% by weight of the particles have an esd less than 0.25 μm and (ii) have a shape factor of at least 45.
It is known to replace part of the processed kaolin clay in a paper coating pigment by particulate calcium carbonate.
Particulate calcium carbonate can be obtained from natural sources or can be manufactured synthetically. Manufactured calcium carbonate is generally obtained by precipitation from aqueous solution. Precipitated calcium carbonate (PCC) is obtained in three different principal crystal forms: the vaterite form, which is thermodynamically unstable, the calcite form which is the most stable and is also the most abundant natural crystalline form, and the aragonite form which is metastable under normal ambient conditions of temperature and pressure, but converts to calcite at elevated temperatures.
The aragonite form typically crystallises as long, thin needles (acicular shape) having a typical length:diameter ratio of about 10:1, but the calcite form exists in several different shapes, of which the most commonly found are: the rhombohedral shape, in which the length and diameter of the crystals are approximately equal and the crystals may be either aggregated or unaggregated; and the scalenodedral shape, in which the crystals are like double, two-pointed pyramids having a typical length:diameter ratio of about 4:1, and which are generally aggregated. All these forms of calcium carbonate can be prepared by carbonation of an aqueous lime-containing medium by suitable variation of the process conditions.
Calcium carbonate can be ground to obtain particulate ground calcium carbonate (GCC), by methods which are well known in the art. GCC particles have a generally spherical form.
Blends of kaolin clay and aragonitic PCC for use in paper coating are known in the art. In the early 1960s, Hagemeyer carried out work on various pigment blends including kaolin/aragonite blends (TAPPI, March 1960, Vol. 43, No. 3, pages 277-288; and TAPPI, February 1964, Vol. 47, No. 2, pages 75-77). Crawshaw et al, 1982 TAPPI Coating Conference Proceedings 143-164 (1982) describes the effect of PCC shape on certain properties of kaolin-PCC paper coating blends. U.S. Pat. No. 5,833,747 (Bleakley et al.) also describes various kaolin clay/aragonite blends in which the aragonite is made by a particular method in which the PCC-containing suspension is at least partially dewatered and subjected to comminution by high shear attrition grinding with an attrition grinding medium. WO-A-00/66509 and WO-A-00/66510 (Lyons et al.) describe various kaolin clay/PCC blends, in which “blocky” kaolin clay is used, by which is stated to mean a shape factor less than 20. The disclosures of all these references are incorporated herein by reference.